Large Fire Years Research Articles

Potentially limited detectability of short-term changes in boreal fire regimes: a simulation study

Climate change is expected to increase area burned in the boreal plains ecozone of Canada in the early 21st century (2001–50). I examined the influence of inter-annual variability in area burned and short observed time series on the probability of detecting if an increase has occurred, using a null model of present and future fire regimes. A wide range of fire cycles are consistent with annual area burned in the late 20th century (1959–99). Fire cycles estimated from the reciprocal of the average annual burn fraction over a 50-year period are not very precise, and overestimate the fire cycle if years with large annual area burned have not recently occurred. Under the default assumptions, the probability of detecting a doubling of annual area burned during 2001–50 is 73% if it occurred instantaneously, but only 31% if it occurred gradually. Imprecise estimates and uncertainty in the ability to detect changes in fire cycles poses challenges for implementing aspects of sustainable forest management. Alternate empirical or model-based statistics, such as return periods for annual areas burned of a given magnitude, may be useful for inferring frequencies and magnitudes of large fire years that have not yet been observed.

Fire History of a Lower Elevation Jeffrey Pine-Mixed Conifer Forest in the Eastern Sierra Nevada, California, USA

For thousands of years, fire has shaped coniferous forests of the western United States. In more recent time, land use practices have altered the role fire plays in the Sierra Nevada. By understanding the past, land managers can design better fuel treatments today. This research explores the fire regimes of Sagehen Experimental Forest in the eastern Sierra Nevada, California, through a fire scar reconstruction of lower elevation Jeffrey pine (Pinus jeffreyi Balf.) and Jeffrey pine-mixed conifer stands. Prehistoric and historic land use practices, fuel accumulation, and climate influenced the fire regime over three periods of time: pre-settlement (1700 to 1859), settlement (1860 to 1925), and suppression (1925 to 2006). Over the period of analysis, 293 fire scars were assigned calendar years. The mean composite fire return interval for all samples in the study area was two years. The mean composite fire return interval was significantly longer for the suppression period than both the pre-settlement and settlement periods. The lack of sufficient fire intervals for analysis using a filter that included fires that scarred at least three or more trees and 25% of the total sample indicates that fires in the study area are small in spatial extent. The proportion of dormant season fires increased from pre-settlement through the suppression period. No fires were recorded as middle and early earlywood during the suppression period. A superposed epoch analysis found significant correlation to warmer (Pacific Decadal Oscillation) and wetter (Palmer Drought Severity Index) conditions three years prior to large fire events during the pre-settlement period. During the post-settlement era, large fire years were correlated to El Niño conditions for two consecutive years prior. Little synchrony of fire events was recorded between fire scarred tree clusters. These findings suggest that small frequent prescribed burns would best mimic the pre-settlement fire regime if fire is reintroduced into the ecosystem.

Influence of Fires on O3 Concentrations in the Western U.S.

Because forest fires emit substantial NOx and hydrocarbons--known contributors to O3 production--we hypothesize that interannual variation in western U.S. O3 is related to the burned area. To evaluate this hypothesis we used a gridded database of western U.S. summer burned area (BA) and biomass consumed (BC) by fires between 101-125 degrees W. The fire data were compared with daytime summer O3 mixing ratios from nine rural Clean Air Status and Trends Network (CASTNET) and National Park Service (NPS) sites. Large fire years exhibited widespread enhanced O3. The summer BA was significantly correlated with O3 at all sites. For each 1 million acres burned in the western U.S. during summer, we estimate that the daytime mean O3 was enhanced across the region by 2.0 ppbv. For mean and maximum fire years, O3 was enhanced by an average of 3.5 and 8.8 ppbv, respectively. At most sites O3 was significantly correlated with fires in the surrounding 5 x 5 degrees and 10 x 10 degrees regions, but not with fires in the nearest 1 x 1 degree region, reflecting the balance between O3 production and destruction in a high NOx environment. BC was a slightly better predictor of O3, compared with BA. The relationship between O3 and temperature was examined at two sites (Yellowstone and Rocky Mountain National Parks). At these two sites, high fire years were significantly warmer than lowfire years; however, daytime seasonal meantemperature and O3 were not significantly correlated. This indicates that the presence of fire is a more important predictor for O3 than is temperature.

Interannual Variations in PM2.5 due to Wildfires in the Western United States

In this study we have evaluated the role of wildfires on concentrations of fine particle (d < 2.5 microm) organic carbon (OC) and particulate mass (PM2.5) in the Western United States for the period 1988-2004. To do this, we examined the relationship between mean summer PM2.5 and OC concentrations at 39 IMPROVE sites with a database of fires developed from federal fire reports. The gridded database of area burned was used to generate a database of biomass fuel burned using ecosystem-specific fuel loads. The OC, PM2.5, and fire data were evaluated for five regions: Northern Rocky Mountains (Region 1), Central Rocky Mountains (Region 2), Southwest (Region 3), California (Region 4), and Pacific Northwest (Region 5). In Regions 1, 2, and 5, we found good correlations of seasonal mean PM2.5 concentrations among the sites within each region. This indicates that a common influence was important in determining the PM concentration at all sites across each region. In Regions 1 and 2, we found a significant correlation between PM2.5 and both the area burned and biomassfuel burned in each region. This relationship is statistically significant using either the area burned or fuel burned, but the correlations are stronger using the biomass fuel burned. In all five regions we found a statistically significant relationship between biomass burned and organic carbon. Using these relationships, we can estimate the amount of PM2.5 due to fires in each region during summer. For the Regions 1 through 5, the average summer-long enhancement of PM2.5 due to fires is 1.84, 1.09, 0.61, 0.81, and 1.21 microg/m3, respectively, and approximately twice these values during large fire years.

Boreal Fire Effects on Subsistence Resources in Alaska and Adjacent Canada

Rural communities in the northern boreal forest depend on a suite of wild species for subsistence, including large game animals, furbearers, fish, and plants. Fire is one of the primary ecological disturbances and determinants of landscape pattern in the northern boreal forest. We review responses of key boreal subsistence species to variation and change in post-fire stand age and other characteristics. Available data for 17 species indicate highly varied post-fire habitat requirements. Mosaics of differing stand ages generated by fire could therefore be most effective in supplying a suite of subsistence species to hunting-and-gathering communities. Recent and projected increases in frequency of large-fire years might produce a more homogeneous landscape dominated by younger stand ages. Increases in fire suppression around communities might also produce a more homogenous landscape, but dominated by older stand ages. Neither of these scenarios provides mixed habitats for a diverse suite of subsistence species. We suggest that one aspect of the complex suite of human-fire interactions is an understanding of wildlife and plant species' response to fire in a subsistence region, in an effort to sustain ecosystem services critical to human well-being.

Introduction. The boreal forest and global change

The boreal forest is the second largest biome in the world containing 33% of the Earth's forest cover ([FAO 2001][1]) of which approximately 25% is natural. It is circumpolar and shares similar taxa across its range. It has approximately 20 300 identified species. Along with the tropics, the

Fire‐climate interactions in forests of the American Pacific coast

We investigate relationships between climate and wildfire activity between 1929 and 2004 in Pacific coast forests of the United States. Self‐Organizing Mapping (SOM) of annual area burned in National Forests (NF) in California, Oregon, and Washington identifies three contiguous NF groups and a fourth group of NF traversed by major highways. Large fire years in all groups are dry compared to small fire years. A sub‐hemispheric circulation pattern of a strong trough over the North Pacific and a ridge over the West Coast is characteristic of large fire years in all groups. This pattern resembles the Pacific North American (PNA) teleconnection and positive phase of the Pacific Decadal Oscillation (PDO). A reverse PNA and negative PDO phase characterizes small fire years. Despite the effect of fire suppression management between 1929 and 2004, forest area burned is linked to climatic variations related to large‐scale atmospheric circulation patterns.

ENSO AND PDO VARIABILITY AFFECT DROUGHT-INDUCED FIRE OCCURRENCE IN ROCKY MOUNTAIN SUBALPINE FORESTS

Understanding the effect of variation in climate on large-fire occurrence across broad geographic areas is central to effective fire hazard assessment. The El Niño– Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) affect winter temperature and precipitation regimes in western North America through mid-latitude teleconnections. This study examines relationships of ENSO and the PDO to drought-induced fire occurrence in subalpine forests of three study areas across the Rocky Mountains: Jasper National Park (JNP, northern Rockies), Yellowstone National Park (YNP, central Rockies) and Rocky Mountain National Park (RMNP, southern Rockies) over the 1700–1975 period. Large-scale climatic anomalies captured by ENSO (NIÑO3) and PDO indices had differential effects on large-fire occurrence across the study areas. Superposed epoch analysis (SEA) showed that large fires in RMNP occurred during extreme La Niña years, while the PDO, although predominantly negative during fire years, did not depart significantly from the mean. In YNP and JNP, neither ENSO nor PDO indices were significantly different from the mean during large-fire years, although fires tended to occur during El Niño and positive PDO years. Constructive phases (years of combined warm [positive] or cool [negative] phases) of ENSO and the PDO were significantly associated with large-fire occurrence across the Rockies, even though these large-scale climatic anomalies were not significant when considered singly in SEAs. Combined warm phases (positive PDO during El Niño) co-occurred with large fires in the central and northern Rockies, while the combined cool phases (negative PDO during La Niña) appeared to promote large fires in the southern Rockies. Almost 70% of large fires in RMNP burned during La Niña events that coincided with a negative PDO, although these phases co-occurred during only 29% of the 1700– 1975 period. Spatial teleconnection patterns between drought, PDO and ENSO across western North America independently support the sign and strength of relationships between these climatic anomalies and subalpine fire occurrence along a broad north–south gradient of the Rockies. Forecasts of ENSO that are dependent on the expected PDO phase suggest promise for fire hazard prediction across the West.

The use of ATSR active fire counts for estimating relative patterns of biomass burning – a study from the boreal forest region

Satellite fire products have the potential to construct inter‐annual time series of fire activity, but estimating area burned requires considering biases introduced by orbiting geometry, fire behavior, and the presence of clouds and smoke. Here we evaluated the performance of fire counts from the Advanced Thermal Scanning Radiometer (ATSR) for the boreal forest region using area burned information from other sources. We found ATSR detection rate varied between regions and different years, being higher during large fire years than during small fire years. The results show ATSR fire counts do not represent an unbiased sample of fire activity, and independent validation may be required prior to using this data set in studies of global emissions from biomass burning.

Wildfires in the southern Canadian Rocky Mountains and their relationship to mid-tropospheric anomalies

In this paper we present evidence for a large-scale (synoptic-scale) meteorological mechanism controlling the fire frequency in the southern Canadian Rocky Mountains. This large-scale control may explain the similarity in average fire frequencies and timing of change in average fire frequencies for the southern Canadian Rocky Mountains. Over the last 86 years the size distribution of fires (annual area burned) in the southern Canadian Rockies was distinctly bimodal, with a separation between small- and large-fire years at approximately 10–25 ha annual area burned. During the last 35 years, large-fire years had significantly lower fuel moisture conditions and many mid-tropospheric surface-blocking events (high-pressure upper level ridges) during July and August (the period of greatest fire activity). Small-fire years in this period exhibited significantly higher fuel moisture conditions and fewer persistent mid-tropospheric surface-blocking events during July and August. Mid-tropospheric surface-blocking events during large-fire years were teleconnected (spatially and temporally correlated in 50 kPa heights) to upper level troughs in the North Pacific and eastern North America. This relationship takes the form of the positive mode of the Pacific North America pattern.